In the Shadow of the Oldwalls

 is a quest in Tyranny.

Synopsis
Raetommon has taken both the Magebane Helm and Forge Master Zdenya. Arri wanted Zdenya as part of their alliance, so pursue Raetommon and rescue the Forge-Bound.

Walkthrough

 * Head to Deserter's March or Twin Rivers.
 * At Deserter's March, you will find Locke and Myrek arguing directly north of your starting location. You can tell them both to stand down and, surprisingly, they will. Talking to Myrek will reveal that he's a fervent believer in Raetommon. You can attack or ask him to explain his position. He believes that all the men Raetommon killed were traitors. With Lore 48, you can point out that the Brotherhood must be rife with them. If you have already visited Twin Rivers, you can also give testimony that Raetommon unleashed the Bane on his own men. Finally, if you left the Bronze Brotherhood in control of Lethian's Crossing during the Conquest, you can state that Raetommon's attack killed many of the garrison.
 * Speaking to Locke will result in him attempting to convince Myrek to go against Raetommon. With the lore check and the two testimonies, this will succeed . If not, you will have to kill him. Either way, take the keystone.
 * At Twin Rivers, it turns out that the mercenaries are fighting the Bane. Brother Caelus pleads with you for help. As a rebel, you can help the Brotherhood defeat the enemy.
 * Find Welby on the other side of the river, to the northeast. She will reveal that Raetommon had gone inside the Oldwalls at Twin Rivers. If you do not already have the keystone, she will ask you to go to Deserter's March to retrieve it.
 * You can recruit her as a follower to participate in your attack on Raetommon, giving you the ability to later recruit the Brotherhood to your cause.
 * Use the Oldwalls Keystone on the entrance and go in. This completes the quest.